Cross-Reference to Related Application
This application is related to a co-pending patent application titled "Method For Controlling The Amplitude Of An Optical Signal", by F. L. Bosch and T. V. Nguyen, Ser. No. 08/136,503, filed simultaneously with, and assigned to the same assignee, as this application
Field of the Invention
The present invention relates to an amplitude detection scheme for an optical transmitter and, more particularly, to a detection scheme useful in providing control of laser modulation over the lifetime of the optical transmitter.
Description of the Prior Art
A continuing concern in the field of laser-based optical transmitters is the change in laser characteristics with temperature and aging. The change in laser characteristics manifests itself as a combination of a shift in the laser threshold, (the current at which the laser begins lasing), and the L-I slope (the light output L vs. current I characteristic). As a result, special operating strategies for controlling the laser bias and modulation currents have been developed. In most cases, a photodiode is mounted in the same package as the laser and a portion of the light exiting the rear face of the laser is captured by the photodiode and used to monitor the laser performance. In particular, a feedback loop comparing the photodiode current to a reference signal maintains the average light output at a desired level by automatically adjusting the bias current. The reference signal is referred to here as a "mark density" reference signal, representing the average density over time of pulses which the laser converts into light. See, for example, an article entitled "GaAlAs Laser Transmitters for Lightwave Transmission Systems", appearing in the Bell System Technical Journal, Vol. 57, No. 6, July-August 1978, beginning at page 1823 and included herein by reference.
During initial use, and as long as only the laser threshold exhibits changes as a function of aging, the above photodiode monitor arrangement is suitable. However, when the laser L-I slope begins to change as a function of age, the light amplitude (and with it, the critical extinction ratio of ON-light to OFF-light) will change. Thus, a need remains for a means of controlling both the average light output and the light amplitude.
One arrangement proposed to provide this need is described in an article entitled "Laser Level Control for High Bit Rate Optical Fibre Systems", by D. W. Smith et al. and presented at the 13th Circuits and Systems International Symposium, Houston, Tex., April 1980 (appearing at pages 926-30 of the Proceedings). In this case, a low-frequency ON-state slope sensing arrangement, utilized in a high bit rate transmitter, permits indirect amplitude control. However, the arrangement as proposed works only with lasers having an exceptionally linear L-I slope, irrespective of temperature or aging. Typical lasers, however, have non-linear L-I slope which may render this approach unusable.
Thus, a need remains for a more robust arrangement for monitoring both average light and light amplitude in an optical transmitter that is usable with non-ideal lasers.